1. Field of the Invention
The present invention relates to sleep mode synchronization in a wireless communication system. More particularly, the present invention relates to an apparatus and method for controlling inconsistent state synchronization which occurs when a Mobile Station (MS) in a sleep mode is not able to receive a traffic indication message in a wireless communication system.
2. Description of the Related Art
In a next generation communication system, also known as the 4th Generation (4G) communication system, research is actively in progress to provide users with services having various Quality of Service (QoS) at a high data rate. In particular, research on the 4G communication system is actively in progress to ensure mobility and QoS in a wireless communication system, such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
In general, power consumption of a Mobile Station (MS) employed in the wireless communication system is considered a primary performance indicator for overall system performance. Therefore, wireless communication system standards such as the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard define a sleep mode and an active mode of the MS in order to minimize the power consumption of the MS.
FIG. 1 is a schematic view illustrating a sleep mode operation defined in an IEEE 802.16 system according to the related art.
Referring to FIG. 1A, an MS of the IEEE 802.16e system can transition to a sleep mode only after receiving a state transition grant from a Base Station (BS). In this case, while granting the state transition of the MS to the sleep mode, the BS can buffer or drop a packet to be transmitted. Further, the BS has to report a presence or absence of data to be transmitted to the MS during a listening interval or a listening window of the MS, and the MS has to determine the presence or absence of the data to be transmitted to the MS by waking up from the sleep mode. In FIG. 1, traffic indication messages, i.e., MOBile TRaFfic INDicator (MOB_TRF-IND) messages 100, 102, and 104, are transmitted to report the presence or absence of packet data to be transmitted. The MOB_TRF-IND messages 100 and 102 including ‘negative traffic indication’ indicate the absence of data, and the MOB_TRF-IND message 104 including ‘positive traffic indication’ indicates the presence of data.
If the presence of data to be transmitted to the MS is determined during the listening interval or the listening window, the MS transitions to an active mode to receive the data from the BS. Thereafter, the MS transmits and receives SLeeP-REQuest/ReSPonse (SLP-REQ/RSP) messages to and from the BS to transition from the active mode to the sleep mode. As a result, a signaling overhead may occur.
As a measure for preventing the signaling overhead, as illustrated in FIG. 1B, the MS may not transition to the active mode even if traffic exists. There is a restriction that the transition between the active mode and the sleep mode has to be achieved by MOB_SLP-REQ/RSP signaling. Accordingly, when the MS has to receive data, the MS receives the data by transitioning between a sleep state and an active state (or an awake state) in a repetitive manner while operating in the sleep mode according to a sleep cycle.
Referring to FIG. 1B, during a listening interval or a listening window in the sleep state, the MS receives MOB_TRF-IND messages 100, 102, 104, 110, and 112 from the BS. Upon receiving the MOB_TRF-IND message 100 including ‘negative traffic indication’ during a listening window of an initial sleep cycle, the MS increases a next sleep cycle to double the initial sleep cycle. Thereafter, upon receiving the MOB_TRF-IND message 102 including ‘negative traffic indication’ after the next sleep cycle elapses, the MS sets the next sleep cycle to double the current sleep cycle. Upon receiving the MOB_TRF-IND message 104 including ‘positive traffic indication’ after the sleep cycle elapses, the MS receives downlink data during a listening window in which the MOB_TRF-IND message 104 is received, and resets a next sleep cycle to a length of the initial sleep cycle. Thereafter, upon receiving the MOB_TRF-IND message 110 including ‘negative traffic indication’ during a listening window after elapse of the sleep cycle which is reset to the initial sleep cycle, the MS increases the next sleep cycle to double the initial sleep cycle. Further, upon receiving the MOB_TRF-IND message 112 including ‘negative traffic indication’ during a listening window after elapse of the increased sleep cycle, the MS increases the next sleep cycle to double the current sleep cycle.
If the MS correctly receives traffic indication information using the MOB_TRF-IND messages transmitted by the BS, sleep cycle synchronization is maintained between the MS and the BS. However, if the MS fails to receive the MOB_TRF-IND messages due to channel state deterioration or the like, the MS cannot know whether the BS transmits ‘positive traffic indication’ or ‘negative traffic indication’. As a result, the MS cannot correctly set a length of a next sleep cycle. Accordingly, sleep cycle synchronization is inconsistent between the MS and the BS.
FIG. 2 illustrates a situation where sleep cycle synchronization is inconsistent in an IEEE 802.16 system according to the related art.
Referring to FIG. 2, an MS correctly receives MOB_TRF-IND messages 200 and 202 including ‘negative traffic indication’ in a periodic manner during listening windows of sleep cycles, and fails to receive a MOB_TRF-IND message 204 including ‘negative traffic indication’ during a listening window after a next sleep cycle elapses. In this case, a BS recognizes a listening window of a next sleep cycle as a time t1 212. However, since the MS fails to receive the MOB_TRF-IND message 204, the MS cannot determine whether to reset the next sleep cycle to an initial sleep cycle according to ‘positive traffic indication’ or to double the next sleep cycle according to ‘negative traffic indication’. A listening window start time of an actual sleep cycle is the time t1 212. However, if the MS resets the next sleep cycle to the initial sleep cycle, synchronization for a listening window of the sleep cycle is inconsistent. Accordingly, there is a need for a method of controlling inconsistent sleep cycle synchronization when a MOB_TRF-IND message is not received in a wireless communication system.